FROG: NERVOUS SYSTEM AND SENSE ORGANS 81 



the size of the pupil can be altered. (4) Immediately 

 behind the iris lies a firm, transparent, subspherical body, 

 the lens, which serves to focus upon the sensitive surface 

 at the back of the eye the light which enters through the 

 pupil. (5) The sensitive surface is provided by the retina, 

 a delicate membrane containing two primary layers, an 

 outer pigment layer of pigmented cells lining the choroid, 

 and an inner retina proper which has at the back of the 

 eye a very complicated structure (Fig. 48, B) and is con- 

 nected with the optic nerve, by which the impulses which 

 give rise to sight are 

 conveyed to the brain. 

 The fibres of the optic 

 nerve pass right 

 through the retina and 

 spread out over its 

 inner surface (that 

 which is turned to- 

 wards the hollow of 

 the eyeball). The per- 

 cipient cells are on the 

 outer surface, against 

 the pigment layer, 

 so that light must 

 pass through the layer FlG -43-— A diagram of the ear of the frog. 





— lab. 



Of nerve fibres to reach col -< Columella ;f.o., fenestra ovahs; Eu., Eusta- 

 chian tube ; lab., part of the membranous 

 labyrinth, containing endolymph ; m., mouth ; 

 Kid., mandible ; peril., perilymph ; sk., skull ; 

 tym., tympanic membrane. 



them. In the front 

 half of the eye the 

 retina loses its com- 

 plicated structure and becomes very thin, but it continues 

 to line the posterior chamber up to the edge of the pupil. 

 The essential part of the ear is the membranous labyrinth 



which we have already mentioned (pp. 33, 75). 



It lies in the cavity of the auditory capsule. 

 This cavity contains a fluid known as perilymph, and the 

 membranous labyrinth contains a fluid known as endolymph. 

 The labyrinth consists of the vestibule and the semicircular 

 canals. The vestibule has an upper, larger division, the 

 utriculus, and a lower, smaller sacculus. From the former 

 arise the three semicircular canals, which are arched tubes 

 opening into the utriculus at both ends. They are placed 

 6 



